1. Field of the Invention
The present invention relates to a digital FM demodulation circuit which uses an arctangent circuit having ROM tables.
2. Description of the Related Art
FIG. 1 is a schematic block diagram showing a conventional digital FM demodulation circuit. The digital FM demodulation circuit includes delay circuits 1, 2, and 6, adders 3 and 7, a divider 4, and an arctangent circuit 5. The operation of the digital FM demodulation circuit will be described. In the demodulation circuit, there is inputted a digital FM signal into which an analog FM signal is converted with a sampling frequency that is four times higher than a frequency of the analog signal.
When the sampling frequency is set to be four times as large as the frequency of the original signal, it is possible to set a phase difference between the sampling frequency and an non-modulated digital FM signal at 90 degrees. Each time when a digital FM signal is passed through a delay circuit, the phase of the signal is delayed at 90 degrees. The sampled digital FM signal passes through a delay circuit 1 to be converted to a second signal that is delayed for a constant time. The second signal is referred to as sin.omega.t.
On the other hand, the sampled digital FM signal is converted also to a third signal which is always phase-shifted at 90 degrees with respect to the second signal, by a Hilbert conversion circuit including the delay circuits 1 and 2, the adder 3, and the divider 4. The third signal is referred to as cos.omega.t. A FM signal C which is phase-shifted by 90 degrees, i.e., cos.omega.t, and A FM signal S which is not phase-shifted, i.e., sin.omega.t are inputted to the arctangent circuit 5. The arctangent circuit 5 performs a division of the two signals to obtain tan.omega.t (=S/C=sin.omega.t/cos.omega.t), and obtains arctan(S/C) from ROM tables. As a result, a phase .omega. of the FM signal is detected.
An output of the arctangent circuit 5 is delayed for one sampling period by the delay circuit 6, and then inverted by an inverting circuit which is not shown. The inverted output is supplied to one input of the adder 7. On the other hand, to the other input of the adder 7, the output of the arctangent circuit 5 is directly inputted. In the adder 7, the two inputs are added to each other, so that a FM detector output as a demodulated signal is obtained.
As described above, a general digital FM demodulation circuit requires an arctangent circuit in which the phase of a sampled signal is calculated from the amplitude of the signal. Usually, it is often that such a calculation circuit itself is not configured and the phase is obtained by using ROM tables corresponding to an input value.
In most of A/D converter circuits, the output digital value is expressed by one bit in code information and n bits in amplitude information. In the following description, therefore, a 4-bit A/D converter in which code information consists of 1 bit and amplitude information of 3 bits is used as an example, and it is assumed that an analog FM signal is converted to a digital FM signal by the A/D converter and the digital FM signal is inputted to a digital FM demodulation circuit. In the 4-bit A/D converter, the amplitude of the input signal is expressed by one of digital values of 15 kinds from -7 to 7. Consequently, each of the above-mentioned S and C can have one of the values from -7 to 7, and 113 ROM tables of arctangents of S/C are necessary (when S/C.gtoreq.0, 64 ROM tables, and when SIC&lt;0, 49 ROM tables). When no countermeasure is taken, therefore, there arises a problem in that the cost of preparing ROM tables is too high.